U.S. patent number 8,199,696 [Application Number 09/823,015] was granted by the patent office on 2012-06-12 for method and apparatus for power control in a wireless communication system.
This patent grant is currently assigned to QUALCOMM Incorporated. Invention is credited to Jack M. Holtzman, Sandip Sarkar.
United States Patent |
8,199,696 |
Sarkar , et al. |
June 12, 2012 |
Method and apparatus for power control in a wireless communication
system
Abstract
Method and apparatus for power control in a packet-data switched
communication system by adapting a transmission energy setpoint to
transmission quality and adjusting the retransmission energy
setpoint accordingly. In one embodiment, the retransmission energy
setpoint is adjusted as a function of retransmission quality.
Inventors: |
Sarkar; Sandip (San Diego,
CA), Holtzman; Jack M. (San Diego, CA) |
Assignee: |
QUALCOMM Incorporated (San
Diego, CA)
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Family
ID: |
25237568 |
Appl.
No.: |
09/823,015 |
Filed: |
March 29, 2001 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20020167907 A1 |
Nov 14, 2002 |
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Current U.S.
Class: |
370/328 |
Current CPC
Class: |
H04W
52/286 (20130101); H04W 52/12 (20130101); H04L
1/1867 (20130101); H04W 52/20 (20130101); H04W
52/48 (20130101) |
Current International
Class: |
H04W
4/00 (20060101) |
Field of
Search: |
;370/235,252,310.2,317,318,328,332,333,338,342 |
References Cited
[Referenced By]
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Other References
US. Appl. No. 09/426,016, entitled "Method and Apparatus for
Minimizing Total Transmission Energy in a Communication System
Employing Retransmission of Frame Received in Error," filed Oct.
22, 1999; Jack Holtzman, et al., Qualcomm Incorporated, San Diego,
California. cited by other .
G.D. Mandyam "Power Control Based on Radio Link Protocol in
cdma2000," IEEE Wireless Communications and Networking Conference,
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Primary Examiner: Ly; Anh-Vu
Attorney, Agent or Firm: Patel; Rupit M.
Claims
What is claimed is:
1. A transmitter, comprising: a processor operative to control an
initial transmission and retransmission of data; and a memory
storage device operative for storing a plurality of
computer-executable instructions to be executed by the processor
comprising: a first set of instructions for receiving an initial
transmission frame error rate and a retransmission frame error rate
from a receiver in response to one or more previously transmitted
frames; a second set of instructions for determining and adjusting
an initial transmission energy setpoint as a function of the
initial transmission frame error rate and an initial transmission
quality and exclusive of any retransmission frame error rate,
wherein the determination of the initial transmission energy
setpoint is responsive to an update trigger; and a third set of
instructions for determining and adjusting a retransmission energy
setpoint as a function of the retransmission frame error rate and a
retransmission quality, wherein the determination of the
retransmission energy setpoint is responsive to the update
trigger.
2. The transmitter of claim 1, wherein the initial transmission
quality is measured by a received error indication signal.
3. The transmitter of claim 1, wherein the initial transmission
energy setpoint and the retransmission energy setpoint are
determined as traffic to pilot ratios.
4. The transmitter of claim 1, wherein the third set of
instructions determines the retransmission energy setpoint as a
function of the retransmission frame error rate, the retransmission
quality, and the initial transmission energy setpoint.
5. The transmitter of claim 4, wherein the third set of
instructions determines the retransmission energy setpoint by
adding a delta value to the initial transmission energy
setpoint.
6. In a wireless communication system, a method comprising:
determining an initial transmission energy setpoint to achieve an
initial transmission frame error rate in an initial transmission of
data; repeatedly adjusting the initial transmission energy setpoint
on occurrence of an initial transmission error in the initial
transmission at a processor and exclusive of any retransmission
frame error rate, wherein the initial transmission error is
received from a receiver; determining a retransmission energy
setpoint to achieve a retransmission frame error rate in a
retransmission of the data; and adjusting the retransmission energy
setpoint on occurrence of a retransmission error in the
retransmission, wherein the retransmission error is received from
the receiver.
7. The method of claim 6, wherein adjusting the retransmission
energy setpoint further comprises: adjusting the retransmission
energy setpoint as a function of the initial transmission energy
setpoint.
8. The method of claim 6, wherein adjusting the retransmission
energy setpoint further comprises: adjusting the retransmission
energy setpoint to achieve a desired frame error rate for
retransmission.
9. The method of claim 6, wherein repeatedly adjusting the initial
transmission energy setpoint further comprises: adjusting the
initial transmission energy setpoint to achieve a desired frame
error rate for transmission.
10. The method of claim 6, wherein the initial transmission frame
error rate is greater than the retransmission frame error rate.
11. The method of claim 6, wherein the initial transmission frame
error rate and the retransmission frame error rate result in a
desired total frame error rate.
12. The method of claim 6, wherein the initial transmission frame
error rate and the retransmission frame error rate are
predetermined values.
13. The method of claim 6, wherein the initial transmission frame
error rate and the retransmission frame error rate are dynamic
values.
14. A non-transitory computer-readable medium encoded with computer
executable instructions, comprising: a first set of instructions
for receiving an initial transmission frame error rate and a
retransmission frame error rate from a receiver in response to one
or more previously transmitted frames; a second set of instructions
for determining and adjusting an initial transmission energy
setpoint as a function of the initial transmission frame error rate
and an initial transmission quality and exclusive of any
retransmission framer error rate, wherein the determination of the
initial transmission energy setpoint is responsive to an update
trigger; and a third set of instructions for determining and
adjusting a retransmission energy setpoint as a function of the
retransmission frame error rate and a retransmission quality,
wherein the determination of the retransmission energy setpoint is
responsive to the update trigger.
15. The computer-readable medium of claim 14, wherein the initial
transmission quality is measured by a received error indication
signal.
16. The computer-readable medium of claim 14, wherein the initial
transmission energy setpoint and the retransmission energy setpoint
are determined as traffic to pilot ratios.
17. The computer-readable medium of claim 14, wherein the third set
of instructions determines the retransmission energy setpoint as a
function of the retransmission frame error rate, the retransmission
quality, and the initial transmission energy setpoint.
18. An apparatus, comprising: means for determining an initial
transmission energy setpoint to achieve an initial transmission
frame error rate in a an initial transmission of data; means for
repeatedly adjusting the initial transmission energy setpoint on
occurrence of an initial transmission error in the initial
transmission and exclusive of any retransmission frame error rate,
wherein the initial transmission error is received from a receiver;
means for determining a retransmission energy setpoint to achieve a
retransmission frame error rate In a retransmission of the data;
and means for adjusting the retransmission energy setpoint on
occurrence of a retransmission error in the retransmission, wherein
the retransmission error is received from the receiver.
19. The apparatus of claim 18, wherein the means for adjusting the
retransmission energy setpoint further comprises: means for
adjusting the retransmission energy setpoint as a function of the
initial transmission energy setpoint.
20. The apparatus of claim 18, wherein the means for adjusting the
retransmission energy setpoint further comprises: means for
adjusting the retransmission energy setpoint to achieve a desired
frame error rate for retransmission.
21. The apparatus of claim 18, wherein the means for repeatedly
adjusting the initial transmission energy setpoint further
comprises: means for adjusting the initial transmission energy
setpoint to achieve a desired frame error rate for transmission.
Description
CO-PENDING RELATED APPLICATIONS FOR PATENT
The present Application for Patent is related to U.S. Pat. No.
6,137,840 entitled "METHOD AND APPARATUS FOR PERFORMING FAST POWER
CONTROL IN A MOBILE COMMUNICATION SYSTEM," issued Oct. 24, 2000;
and U.S. patent application Ser. No. 09/426,016 entitled "METHOD
AND APPARATUS FOR MINIMIZING TOTAL TRANSMISSION ENERGY IN A
COMMUNICATION SYSTEM EMPLOYING RETRANSMISSION OF FRAME RECEIVED IN
ERROR," filed Oct. 22, 1999; each assigned to the assignee hereof
and each expressly incorporated herein by reference.
BACKGROUND
1. Field
The present method and apparatus relate generally to communication,
and more specifically to power control in a wireless communication
system.
2. Background
Increasing demand for wireless data transmission and the expansion
of services available via wireless communication technology have
led to the development of systems capable of handling voice and
data services. One spread spectrum system designed to handle the
various requirements of these two services is a Code Division
Multiple Access, CDMA, system referred to as cdma2000, which is
specified in "TIA/EIA/IS-2000 Standards for cdma2000 Spread
Spectrum Systems." Enhancements to cdma2000 as well as alternate
types of voice and data systems are also in development.
As the amount of data transmitted and the number of transmissions
increase, the limited bandwidth available for radio transmissions
becomes a critical resource. There is a need, therefore, for an
efficient and accurate method of transmitting information in a
communication system that optimizes use of available bandwidth.
SUMMARY
Embodiments disclosed herein address the above stated needs by
providing in a wireless communication system, a method of power
control that determines an energy setpoint to achieve a
transmission frame error rate, adjusts the energy setpoint on
occurrence of a transmission error, determines a retransmission
energy setpoint to achieve a retransmission frame error rate, and
adjusts the retransmission energy setpoint on occurrence of a
retransmission error.
In one aspect, a base station apparatus includes a processor
operative to control transmission and retransmission of data, and a
memory storage device operative for storing a plurality of
computer-readable instructions. The instructions include a first
set of instructions for determining a transmission frame error rate
and a retransmission frame error rate, a second set of instructions
for determining a transmission energy setpoint as a function of the
transmission frame error rate and the transmission quality, and a
third set of instructions for determining a retransmission energy
setpoint as a function of the retransmission frame error rate and
the retransmission quality. In one embodiment, the transmission
quality is measured by a received error indication signal, wherein
the error indication signal may be an error indication bit.
According to another embodiment, the third set of instructions
determines the retransmission energy setpoint as a function of the
retransmission frame error rate, the retransmission quality, and
the transmission energy setpoint, such as by maintaining a delta
value between the transmission energy setpoint and the
retransmission energy setpoint.
In another aspect, a method in a wireless communication system
includes determining a transmission energy setpoint to achieve a
transmission frame error rate, adjusting the transmission energy
setpoint on occurrence of a transmission error, determining a
retransmission energy setpoint to achieve a retransmission frame
error rate, and adjusting the retransmission energy setpoint on
occurrence of a retransmission error. In one embodiment, adjusting
the transmission energy setpoint further includes adjusting the
retransmission energy setpoint as a function of the transmission
energy setpoint. In another embodiment, adjusting the
retransmission energy setpoint further includes adjusting the
retransmission energy setpoint to achieve a desired frame error
rate for retransmission.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of a channel architecture in a wireless
communication system.
FIG. 2 is a diagram of a wireless communication system.
FIG. 3 is a diagram of a transmission scenario in a wireless
communication system.
FIG. 4 is a diagram of transmission and retransmission scenarios
and in a wireless communication system.
FIG. 5 is a timing diagram illustrating outer loop adjustment of a
closed loop power control method in a wireless system.
FIG. 6 is a timing diagram illustrating the ratio of traffic signal
strength to pilot signal strength in a wireless system.
FIG. 7 is a flow diagram of a method for energy setpoint adjustment
in a wireless communication system.
FIG. 8 is a diagram of an alternate method for energy setpoint
adjustment in a wireless communication system.
FIG. 9 is a diagram of a transceiver in a wireless communication
system.
FIG. 10 is a diagram of a method of energy setpoint adjustment in a
wireless communication system.
DETAILED DESCRIPTION
The word "exemplary" is used exclusively herein to mean "serving as
an example, instance, or illustration." Any embodiment described
herein as "exemplary" is not necessarily to be construed as
preferred or advantageous over other embodiments.
Spread spectrum communication systems, such as Code Division
Multiple Access, CDMA, systems detailed in standards including but
not limited to the "TIA/EIA/IS-95 Mobile Station-Base Station
Compatibility Standard for Dual-Mode Wideband Spread Spectrum
Cellular System," hereinafter referred to as "the IS-95 standard,"
the "TIA/EIA/IS-2000 Standards for cdma2000 Spread Spectrum
Systems," hereinafter referred to as "the cdma2000 standard,"
and/or the "TIA/EIA/IS-856 cdma2000 High Rate Packet Data Air
Interface Specification," hereinafter referred to as "the HDR
standard," spread signals such that multiple signals occupy a same
channel bandwidth, wherein each signal has its own distinct
Pseudorandom Noise, PN, sequence.
Operation of a CDMA system is described in the following U.S.
Patents: U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM
MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL
REPEATERS;" U.S. Pat. No. 5,103,459, entitled "SYSTEM AND METHOD
FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE
SYSTEM;" and U.S. Pat. No. 5,504,773, entitled "METHOD AND
APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION; each
assigned to the assignee of the present Application for Patent and
hereby expressly incorporated by reference.
In a spread spectrum system, multiple users transmit messages
simultaneously over a same channel bandwidth. As the frequency
spectrum is a finite resource, these systems provide methods for
maximizing the use of this resource by sharing the spectrum while
supporting a large number of users with minimal interference. The
extension of these methods to the high speed transmission of data
allows reuse of existing hardware and software. Designers already
familiar with such standards and methods may use this knowledge and
experience to extend these systems to high speed data
transmissions.
In a wireless communication system, including spread spectrum
systems, a mobile unit communicates with landline communication
network(s) through a base station. The mobile unit may be referred
to as a mobile station, remote station, subscriber, access
terminal, etc. The base station may be referred to as an access
network, etc. The mobile station transmits signals to the base
station via a communication link called a Reverse Link, RL, and the
base station sends signals to a mobile station via a communication
link called a Forward Link, FL. On the RL, each transmitting mobile
station or remote station acts as interference to other remote
stations in the network.
As each user transmits to and receives from the base station, other
users are concurrently communicating with the base station. Each
user's transmissions on the RL introduces interference to other
users. To overcome interference in the received signals, a
demodulator seeks to maintain a sufficient ratio of bit energy to
interference power spectral density, referred to as
E.sub.b/N.sub.0, in order to demodulate the signal at an acceptable
probability of error. Power Control, PC, is a process that adjusts
the transmitter power of one or both of the FL and the RL to
satisfy a given error criteria. Ideally, the power control process
adjusts the transmitter power(s) to achieve at least the minimum
required E.sub.b/N.sub.0 at the designated receiver. Still further,
it is desirable that no transmitter uses more than the minimum
E.sub.b/N.sub.0 required to achieve a desired Quality of Service,
QOS. This ensures that any benefit to one user achieved through the
power control process is not at the unnecessary expense of any
other user.
In a CDMA communication system, each user appears as random noise
to other users in the system due to the various spreading codes
used for user identification. Controlling the power of an
individual user reduces interference to others throughout the
system. Without power control, multiple users at different
distances from a common base station would transmit at a same power
level. Transmissions from those users proximate the base station
are then received at the base station having a higher energy
resulting in a Signal-to-Noise Ratio, SNR, disparity between users.
This inequity is termed the "near-far problem." As each user needs
to attain a required SNR level, the near-far problem limits the
capacity of the system. Power control is used to provide smooth
operation in a spread-spectrum system.
Power control impacts the capacity of the system by ensuring that
each transmitter only introduces a minimal amount of interference
to other users; and thus, increases processing gain. Processing
gain is the ratio of the transmission bandwidth, W, to the data
rate, R. The ratio of E.sub.b/N.sub.0 to W/R is related to SNR.
Processing gain overcomes a finite amount of interference from
other users, i.e., total noise. System capacity is, therefore,
proportional to processing gain and SNR. Feedback information is
provided from a receiver to a transmitter as a link quality
measure. The feedback ideally is a fast transmission having low
latency. Power control then uses this feedback information
regarding link quality to adjust transmission parameter(s).
Power control allows the system to adapt to changing conditions
within an environment, including but not limited to the
geographical conditions and mobile velocity. As the changing
conditions impact the quality of a communication link, the
transmission parameters adjust to accommodate the changes. This
process is referred to as "link adaptation." It is desirable for
link adaptation to track the condition(s) of the system as
accurately and quickly as possible.
According to one embodiment, link adaptation is controlled by the
quality of a communication link, wherein the SNR of the link
provides a quality metric for evaluating the link. The SNR of the
link may be measured as a function of Carrier-to-Interference
ratio, C/I, at the receiver. For voice communications, the quality
metric C/I may be used for providing power control commands
instructing the transmitter to either increase or decrease power.
For packet data communications, such as transmitted in an HDR
system as specified in "TIA-856 cdma2000 High Rate Packet Data Air
Interface Specification," 3GPP, and 3GPP2, data communications are
scheduled among multiple users, where at any given time, only one
user receives data from the access network or base station. In a
packet-switched data system, the quality metric measurement, such
as SNR, may provide valuable information to the base station or
access network transmitter in determining proper data rate,
encoding, modulation and scheduling of data communications.
Therefore, it is beneficial to provide the quality metric
efficiently from the remote station to the base station.
To minimize interference and maximize the RL capacity, the transmit
power of each remote station is controlled by three RL power
control loops. The first power control loop, referred to as "open
loop" power control, adjusts the transmit power of the remote
station(s) such that the received power from each user is
approximately equal at the base station. One power control scheme
sets the transmit power inversely proportional to the received
power on the FL. In a system according to one embodiment, transmit
power is given by p.sub.out=-73-p.sub.in, wherein p.sub.in is the
power received by the remote station given in dBm, p.sub.out is the
transmit power of the remote station given in dBm, and -73 is a
constant. Open loop power control is performed at the remote
station and is performed without direction from the base station.
Open loop power control is initiated when a remote station gains
access to the base station and a communication is established. The
operating environment continues to change while a communication is
active; therefore, the path losses experienced on the FL and RL
between the base station and the remote station change as a
function of time.
Open loop power control compensates for slow-varying and lognormal
shadowing effects, wherein there is a correlation between FL and RL
fading. Other effects are frequency-dependent, such as fast
Rayleigh fading and others. Specifically, for a given communication
link, a unique frequency assignment is given to the FL that is
different from the frequency assignment of the RL. Power control
using the received signal from one link alone is not sufficient to
correct frequency-dependent affects on the other link. For example,
the behavior of the FL signals as received at the remote station
does not necessarily provide sufficient information for adjustment
of RL transmissions that are processed at a different frequency. In
other words, open loop power control in isolation will not
compensate for frequency-dependent affects.
Another, or an additional power control mechanism, referred to as
"closed loop" power control, may be used to resolve power
fluctuations due to Rayleigh fading effects, as well as other
frequency-dependent effects. After call establishment, closed loop
power control is used in coordination with open loop power control.
Closed loop power control has an inner loop and an outer loop. The
inner loop uses a predetermined SNR threshold or setpoint to make
power-up and power-down decisions. The outer loop dynamically
adjusts the SNR threshold to maintain a desired link quality.
With respect to the inner loop of the closed loop power control,
the base station continuously monitors the RL and measures the link
quality. For the RL, closed loop power control adjusts the
transmission power of the remote station such that the link
quality, as measured by the
Energy-per-bit-to-noise-plus-Interference ratio E.sub.b/I.sub.0 of
the RL signal received at the base station is maintained at a
predetermined level. This level is referred to as the
E.sub.b/I.sub.0 set point. The base station measures the
E.sub.b/I.sub.0 of the RL signal received at the base station and
transmits a RL power control bit to the remote station on the
forward traffic channel in response to the measured
E.sub.b/I.sub.0. When the measured E.sub.b/I.sub.0 is too high, the
base station instructs the remote station to decrease transmission
power. If the measured E.sub.b/I.sub.0 is too low, the base station
instructs the remote station to increase transmission power. The
instructions are sent on a sub-channel of the FL. In one
embodiment, the power control instructions are sent as power
control bit(s), wherein increases are in +1 dB steps and decreases
are in -1 dB steps. According to this embodiment, the RL power
control bits are sent 16 times per 20 msec frame, or at an 800 bps
rate. The forward traffic channel carries the RL power control bits
along with the data from the base station to the remote
station.
For packet data transmission, the spread spectrum system transmits
packets of data as discrete data frames. The desired level of
performance or link quality is typically measured as a function of
the Frame-Error-Rate, FER. Calculation of FER introduces time
delays in order to accumulate sufficient bits to accomplish the
calculation.
The inner loop power control adjusts the E.sub.b/I.sub.0 set point
such that the desired level of performance, as measured by the FER,
is maintained. The required E.sub.b/I.sub.0 to obtain a given FER
depends upon the propagation conditions. The outer loop power
control adjusts the E.sub.b/I.sub.0 set point in response to
changes in the system.
For packet data transmission, the spread spectrum system transmits
packets of data as discrete data frames. The desired level of
performance or link quality is typically measured as a function of
the FER. Calculation of FER introduces time delays in order to
accumulate bits. The inner loop power control then adjusts the
E.sub.b/I.sub.0 set point such that the desired level of
performance, as measured by the FER, is maintained. The required
E.sub.b/I.sub.0 depends upon the propagation conditions, wherein
the E.sub.b/I.sub.0 is calculated to obtain a given FER. This power
control is often called the outer loop.
On the FL, the transmission power of the base station is controlled
for several reasons. A high transmission power from the base
station can cause excessive interference with the signals received
at other remote stations. Another problem exists for multipaths
received at a mobile station, wherein at least some of the
multipaths are not resolvable into constituent signals. Those
multipaths that are not resolvable create "self-interference."
Alternatively, if the transmission power of the base station is too
low, the remote station can receive erroneous data transmissions.
There may not be sufficient energy for the base station to
communicate with all mobile stations, in particular mobile stations
not proximate the base station. Terrestrial channel fading and
other known factors can affect the quality of the FL signal as
received by the remote station. As a result, each base station
attempts to adjust its transmission power to maintain the desired
level of performance at the remote station.
Power control on the FL is especially important for data
transmissions. Data transmission is typically asymmetric with the
amount of data transmitted on the FL being greater than on the RL.
With an effective power control mechanism on the FL, wherein the
transmission power is controlled to maintain the desired level of
performance, the overall FL capacity can be improved.
In one embodiment, the remote station transmits an
Error-Indicator-Bit, EIB, message to the base station when a
transmitted frame of data is received in error. The EIB can be
either a bit contained in the reverse traffic channel frame or a
separate message sent on the reverse traffic channel. In response
to the EIB message, the base station increases its transmission
power to the remote station.
One disadvantage of this method is the long response time. The
processing delay encompasses the time interval from the time the
base station transmits the frame with inadequate power to the time
the base station adjusts its transmission power in response to the
error message from the remote station. This processing delay
includes the time it takes for: (1) the base station to transmit
the data frame with inadequate power; (2) the remote station to
receive the data frame; (3) the remote station to detect the frame
error (e.g. a frame erasure); (4) the remote station to transmit
the error message to the base station; and (5) the base station to
receive the error message and appropriately adjust its transmission
power. The forward traffic channel frame must be received,
demodulated, and decoded before the EIB message is generated. Then
the reverse traffic channel frame carrying the EIB message must be
generated, encoded, transmitted, decoded, and processed before the
bit can be used to adjust the transmit power of the forward traffic
channel.
Typically, the desired level of performance is one percent Frame
Error Rate, FER. Therefore, on the average, the remote station
transmits one error message indicative of a frame error every 100
frames. In accordance with the IS-95-A standard, each frame is 20
msec long. This type of EIB based power control works well to
adjust the FL transmit power to handle shadowing conditions, but
due to its slow speed does not handle fading conditions as
well.
One method for controlling the FL transmission power utilizes the
E.sub.b/I.sub.0 of the received signal at the remote station. Since
the FER is dependent on the E.sub.b/I.sub.0 of the received signal,
a power control mechanism can be designed to maintain the
E.sub.b/I.sub.0 at the desired level. This design encounters
difficulty if data is transmitted on the FL at variable rates. On
the FL, the transmission power is adjusted depending on the data
rate of the data frame. At lower data rates, each data bit is
transmitted over a longer time period by repeating the modulation
symbol. The energy-per-bit E.sub.b is the accumulation of the
received power over one bit time period and is obtained by
accumulating the energy in each modulation symbol. For an
equivalent amount of E.sub.b, each data bit can be transmitted at
proportionally less transmission power at the lower data rates.
Typically, the remote station does not know the transmission rate a
priori and cannot compute the received energy-per-bit E.sub.b until
the entire data frame has been demodulated, decoded, and the data
rate of the data frame has been determined, wherein the rate is one
power control message per frame. This is in contrast with the RL
approach in which there can be one power control message (bit)
sixteen times per frame as in one embodiment.
At lower rates, the remote station may not transmit continuously.
When the remote station is transmitting, the remote station
transmits at the same power level and the same waveform structure
regardless of the transmission rate. The base station determines
the value of a power control bit and sends this bit to the remote
station sixteen times per frame. Since the remote station knows the
transmission rate, the remote station can ignore power control bits
corresponding to times when it was not transmitting. This permits
fast RL power control. However, the effective power control rate
varies with the transmission rate. For one embodiment, the rate is
800 bps for full rate frames and 100 bps for 1/8 rate frames.
Original CDMA standards have been optimized for transmission of
variable-rate voice frames. In order to support two-way voice
communications, as typified in wireless phone applications, it is
desirable that a communication system provide fairly constant and
minimal data delay. For this reason, many CDMA systems are designed
with powerful Forward Error Correction, FEC, protocols and
vocoders, which are designed to respond gracefully to voice frame
errors. Error control protocols that implement frame retransmission
procedures add unacceptable delays for voice transmission.
Packetizing data allows for increased speed and accuracy of
communication, and is therefore desirable for wireless data
communications. In efforts to integrate wireless and other
communication media with the Internet, an increasing number of
applications are being developed using a standard Internet
Protocol, or IP. This IP is a software standard that describes how
to track Internetwork addresses, route messages, and recognize
incoming messages; thus allowing a packet of data to traverse
various networks on its way from originator to target recipient.
The originator is the mobile unit initiating the communication, and
the target is the desired participant. Within an IP network, each
resource, such as a computer, is assigned an IP address for
identification.
In many non-voice applications, such as the transmission of IP
data, the delay requirements of the communication system are much
less stringent than in voice applications. In the Transmission
Control Protocol, TCP, probably the most prevalent of protocols
used in an IP network, virtually infinite transmission delays are
allowed in order to guarantee error-free transmission. TCP uses
retransmissions of IP datagrams, as IP packets are commonly called,
to provide this transport reliability.
IP datagrams are transmitted in frames, wherein each frame is
defined by a predetermined time duration. Generally, IP datagrams
are too large to fit into a single frame as defined for voice
transmission. Even after dividing an IP datagram into segments
small enough to fit into a set of frames, the entire set of frames
would have to be received without error for the single IP datagram
to be useful to TCP. The targeted FER typical of a CDMA voice
system makes the probability of error-free reception of all
segments of a single datagram very low.
CDMA standards provide for such alternative service options, data
services for example, to enable the transmission of other types of
data in lieu of voice frames. In one embodiment, a Radio Link
Protocol, RLP, incorporates an error control protocol with frame
retransmission procedures over a CDMA frame layer. RLP is of a
class of error control protocols known as Negative
Acknowledge-based, or NAK-based, Automatic Repeat Request, or ARQ,
protocols, which are well known in the art. The RLP facilitates the
transmission of a byte-stream, rather than a series of voice
frames, through a CDMA communication system.
FIG. 1 illustrates an architectural layering 10 of an exemplary
embodiment of a wireless system protocol. The physical layer 12
indicates the channel structure, frequency, power output,
modulation type, and encoding specifications for the forward and
RLs. The Medium Access Control, MAC, layer 14 defines the
procedures used to receive and transmit over the physical layer 12.
For an HDR system, the MAC layer 14 includes scheduling
capabilities to balance users or connections. Such balancing
typically schedules low throughput for channels with poor coverage,
thus freeing up resources allowing high throughput for channels
with good connections. Also, the MAC layer processes transmissions
when a channel has a good connection. The next layer, the Link
Access Control, LAC, layer 16, provides an access procedure for the
radio link. According to one embodiment, the Radio Link Protocol,
RLP, layer 18 provides retransmission and duplicate detection for
an octet-aligned data stream. RLP is of a class of error control
protocols known NAK-based ARQ protocols, which are well known in
the art. In one embodiment, RLP facilitates the transmission of a
byte-stream, rather than a series of voice frames, through a
communication system.
In the context of a packet service, the LAC layer 16 carries
Point-to-Point Protocol packets, PPP packets. The High Level Data
Link Control HDLC layer 20 is a link layer for PPP communications.
Control information is placed in specific patterns, which are
dramatically different from the data in order to reduce errors. The
HDLC layer 20 performs framing of the data prior to PPP processing.
The PPP layer 22 then provides compression, authentication,
encryption and multi-protocol support. The IP layer 24 keeps track
of Internetwork addressing for different nodes, routes outgoing
messages, and recognizes incoming messages.
Protocols running on top of PPP, such as IP layer 24, carry user
traffic. Note that each of these layers may contain one or more
protocols. Protocols use signaling messages and/or headers to
convey information to a peer entity on the other side of the
air-interface. For example, in a High Data Rate, HDR, system,
protocols send messages with a default signaling application.
The architecture 10 is applicable to an Access Network, AN, for
providing data connectivity between an IP network, such as the
Internet, and access terminals, including wireless mobile units.
Access Terminals, ATs, provide data connectivity to a user. An AT
may be connected to a computing device such as a laptop personal
computer, or may be a self-contained data device such as a personal
digital assistant. There are a variety of wireless applications and
an ever increasing number of devices, often referred to as IP
appliances or web appliances.
As illustrated in FIG. 1, layers above the RLP layer 18 are service
network layers and layers below the HDLC layer 20 are radio network
layers. In other words, the radio network layers affect the
air-interface protocols. The radio network layers of the exemplary
embodiment are consistent with those applicable in an HDR system.
HDR generally provides an efficient method of transmitting data in
a wireless communication system. Alternate embodiments may
implement the cdma2000 standard, an IS-95 standard, or other
per-user connection systems, such as the "ANSI J-STD-01 Draft
Standard for W-CDMA (Wideband Code Division Multiple Access) Air
Interface Compatibility Standard for 1.85 to 1.99 GHz PCS
Applications," referred to as "W-CDMA."
As illustrated in FIG. 1, in one embodiment of a wireless protocol,
several protocol layers typically reside above the RLP layer. IP
datagrams, for example, are typically converted into a PPP byte
stream before being presented as a byte stream to the RLP protocol
layer. As the RLP layer ignores the protocol and framing of higher
protocol layers, the stream of data transported by RLP is said to
be a "featureless byte stream."
RLP was originally designed to satisfy the requirements of sending
large datagrams through a CDMA channel. For example, if an IP
datagram of 500 bytes was to be simply sent in frames carrying 20
bytes each, the IP datagram would fill 25 consecutive frames.
Without some kind of error control layer, all 25 of these RLP
frames would have to be received without error in order for the IP
datagram to be useful to higher protocol layers. On a CDMA channel
having a 1% frame error rate, the effective error rate of the IP
datagram delivery would be (1-(0.99).sup.25), or 22%. This is a
very high error rate compared to most networks used for IP traffic.
RLP was designed as a link layer protocol that would decrease the
error rate of IP traffic to be comparable to the error rate typical
of a 10.sup.-2 ethernet channel.
In a spread-spectrum wireless communication system, such as a
cdma2000 system, multiple users transmit to a transceiver, often a
base station, in the same bandwidth at the same time. The base
station may be any data device that communicates through a wireless
channel or through a wired channel, for example, using fiber optic
or coaxial cables. A user may be any of a variety of mobile and/or
stationary devices including but not limited to a PC card, a
compact flash, an external or internal modem, or a wireless or a
wireline phone. A user is also referred to as a remote station.
Note that alternate spread-spectrum systems include, but are not
limited to, systems such as: packet-switched data services;
Wideband-CDMA, W-CDMA, systems, such as specified by Third
Generation Partnership Project, 3GPP; voice and data systems, such
as specified by Third Generation Partnership Project Two,
3GPP2.
FIG. 2 illustrates one embodiment of a wireless communication
system 30, wherein system 30 is a spread spectrum CDMA system
capable of voice and data transmissions. System 30 includes two
segments: a wired subsystem and a wireless subsystem. The wired
subsystem is the Public Switched Telephone Network, PSTN 36, and
the Internet 32. The Internet 32 portion of the wired subsystem
interfaces with the wireless subsystem via Inter-Working Function
Internet, IWF 34. The ever-increasing demand for data
communications is typically associated with the Internet and the
ease of access to the data available thereby. However, advancing
video and audio applications increase the demand for transmission
bandwidth.
The wired subsystem may include but is not limited to other modules
such as an instrumentation unit, a video unit, etc. The wireless
subsystem includes the base station subsystem, which involves the
Mobile Switching Center, MSC 38, the Base Station Controller, BSC
40, the Base Transceiver Station(s), BTS(s) 42, 44, and the Mobile
Station(s), MS(s) 46, 48. The MSC 38 is the interface between the
wireless subsystem and the wired subsystem. It is a switch that
talks to a variety of wireless apparatus. The BSC 40 is the control
and management system for one or more BTS(s) 42, 44. The BSC 40
exchanges messages with the BTS(s) 42, 44 and the MSC 38. Each of
the BTS(s) 42, 44 consists of one or more transceivers placed at a
single location. Each of the BTS(s) 42, 44 terminates the radio
path on the network side. The BTS(s) 42, 44 may be co-located with
BSC 40 or may be independently located.
The system 30 includes radio air interface physical channels 50, 52
between the BTS(s) 42, 44 and the MS(s) 46, 48. The physical
channels 50, 52 are communication paths described in terms of the
digital coding and RF characteristics.
As discussed hereinabove, a FL is defined as a communication link
for transmissions from one of the BTS(s) 42, 44 to one of the MS(s)
46, 48. An RL is defined as a communication link for transmissions
from one of the MS(s) 46, 48 to one of the BTS(s) 42, 44. According
to one embodiment, power control within system 30 includes
controlling transmit power for both the RL and the FL. Multiple
power control mechanisms may be applied to the FL and RL in system
30, including reverse open loop power control, reverse closed loop
power control, forward closed loop power control, etc. Reverse open
loop power control adjusts the initial access channel transmission
power of MS(s) 46, 48, and compensates for variations in path loss
attenuation of the RL. The RL uses two types of code channels:
traffic channel(s), and access channel(s).
Note that for data services a remote station may be referred to as
an AT, wherein an AT is a device providing data connectivity to a
user. An AT may be connected to a computing device, such as a
laptop personal computer, or it may be a self-contained data
device, such as a personal digital assistant. Further, the base
station may be referred to as an AN, wherein the AN is network
equipment providing data connectivity between a packet switched
data network, such as the Internet, and at least one AT. The
reverse access channel is used by ATs to communicate with the AN
when no traffic channel is assigned. In one embodiment there is a
separate reverse access channel for each sector of the AN.
Referring to FIG. 2, each communication channel 50, 52 includes a
FL, carrying information from BTS(s) 42, 44 to MS(s) 46, 48, and a
RL, carrying information from BTS(s) 42, 44 to MS(s) 46, 48.
Information communicated between BTS(s) 42, 44 to MS(s) 46, 48
respectively, is required to meet a predetermined reliability
level. In the exemplary embodiment, the information on FL is
transmitted in frames, and the required reliability level is
expressed as a target FER as received by the MS(s) 46, 48.
One method of achieving the required FER in a system such as system
30 is retransmission of transmitted information. A transmitting
station transmits information, contained in frames, with a first
energy E.sub.1. The transmitted information is received by a
receiving station with a first frame error rate FER.sub.1, wherein
the subscript 1 refers to the first or original transmission. The
receiving station reports the first FER.sub.1 and identity of those
frames received in error back to the transmitting station. The
transmitting station selects a second transmission energy E.sub.2,
and re-transmits the frames received in error. The receiving
station receives the frames with a second frame error rate
FER.sub.2, wherein the subscript 2 refers to the second
transmission. Alternate embodiments may include any number of
retransmission, wherein each retransmission i has an associated
E.sub.1; and FER.sub.i. When the energies E.sub.i and E.sub.2 are
properly selected, the effective FER after the second transmission
will be equal to the target FER. In other words, the total frame
error rate resulting from the transmission and retransmission will
be equal to a target FER. There are an infinite number of
combinations of E.sub.1 and E.sub.2 that would achieve an effective
FER equal to the target FER.
As communications systems, and CDMA communication systems in
particular, are noise limited, it is advantageous to choose E.sub.1
and E.sub.2 in a manner yielding minimum total transmission energy.
The total transmission energy, (E), is equal to the energy used for
the first transmission plus the energy for retransmission of those
frames initially received in error, wherein
(E)=E.sub.1+f(E.sub.1)E.sub.2. E.sub.1 is energy for the first
transmission, E.sub.2 is energy for retransmission, and f(E.sub.1)
is a frame error rate for transmission with energy E.sub.1. The
condition that the effective FER be equal to the target FER can be
expressed as T.sub.FER=f(E.sub.1)f(E.sub.2), wherein T.sub.FER is
the target frame error rate. The effective frame error rate is the
product of f(E.sub.1), a frame error rate for transmission with
energy E.sub.1, and f(E.sub.2), a frame error rate for transmission
with energy E.sub.2.
The task of selecting E.sub.1 and E.sub.2 for minimal total energy
<E>, while assuring that effective FER after the second
transmission will be equal to the target FER, is equivalent to
solving for <E> subject to T.sub.FER. Such solution requires
the knowledge of the FER as a function of energy or a measure of
energy, wherein FER=f(E). The energy measure E may, for example, be
the Energy-per-bit to Noise ratio
##EQU00001## This relationship is a function of several variables,
including, but not limited to, attenuation, fading, the number of
multipaths, the relative velocity of remote station with respect to
the base station, etc.
Retransmission provides error correction in a wireless
communication system that is particularly applicable to packetized
data transmissions. The retransmission may be performed at an
increased energy level with respect to the energy level of the
original transmission. The process of increasing the energy level
used for retransmission is referred to as "power boosting." In one
embodiment, power boosting assumes that the energy level of the
first transmission was not sufficient to achieve the target frame
error rate, and, therefore, increased energy is applied to
subsequent retransmission. Power boosting may reduce the total
energy used to achieve a target FER as compared to retransmission
at a same energy level as the original transmission, i.e., equal
energy case.
As illustrated in FIG. 3, for the case of a single transmission
that satisfies the target FER, the FER.sub.0 corresponds to one
transmission at a power level E.sub.0. At the energy level,
E.sub.0, the transmitted frames are received with an acceptable FER
to allow further processing. In a single transmission scenario, the
power control outer loop adjusts the energy level E.sub.0 in
response to the FER of the received transmission. The FER may be
provided from the receiver back to the base station by way of a FER
message. In one embodiment, the mobile station provides an Error
Indication Bit, EIB, as feedback to the base station.
FIG. 4 illustrates a specification for transmission and
retransmission. In one embodiment, equal energy is used for
transmission and retransmission. The target FER is achieved by
application of FER.sub.1 to the transmission and FER.sub.2 to the
retransmission. The total effective FER is equal to
FER.sub.1*FER.sub.2. On the first transmission the energy level is
set to E.sub.1, while the retransmission applies an energy level
E.sub.2.
According to an equal power scenario, FER.sub.1 is equal to
FER.sub.2, and the corresponding energy levels are equal, i.e.,
E.sub.1=E.sub.2. A target FER is given as FER.sub.1*FER.sub.2. In
this case, the individual energy levels, E.sub.1 and E.sub.2, are
each less than the energy level E.sub.0 of the single transmission
case of FIG. 3.
According to one embodiment, the specified FER values are not
equal, but rather FER.sub.1 is less than FER.sub.2. The lesser
energy is applied to the original transmission in order to reduce
transmission power and is used to achieve the target FER. If the
first transmission achieves the target FER, there is no
retransmission of data. In contrast, if the first transmission does
not achieve the target FER, a retransmission is processed at an
increased energy level E.sub.2. The increase in energy assumes that
E.sub.1 was insufficient to achieve the target FER.
According to one embodiment, E.sub.1 and E.sub.2 are maintained at
a predetermined relation. Power control as illustrated in FIG. 5 is
used to adjust the E.sub.1 to achieve the specified FER.sub.1. In
response, the value of E.sub.2 is calculated based on the adjusted
value of E.sub.1 to maintain the predetermined relation.
Maintaining the relationship between energy setpoints is easily
implemented by software instruction.
In an alternate embodiment, parallel power control loops, such as
illustrated in FIG. 5, are used to adjust E.sub.1 and E.sub.2. The
retransmission frame errors are used to adjust the energy level
E.sub.2, while the transmission frame errors are used to adjust the
energy level E.sub.1. Dynamic adjustment to different FER values
provides improved power control. In this case, a criteria is
applied for updating the FER value(s) such as an update trigger. In
one embodiment, the FER adjustment is triggered on occurrence of an
error. Alternate embodiments may adjust the FER on occurrence of a
predetermined number of errors. Additionally, the adjustments to
E.sub.1 and E.sub.2 may be different allowing the ratio between
them to change. In one embodiment, the step values for incremental
adjustments to adjust E.sub.1 and E.sub.2 are directional, wherein
a first value is used to increment E.sub.1 and second value to
increment E.sub.2. Similarly, according to one embodiment a third
value is used to decrement E.sub.1 and a fourth value to decrement
E.sub.2. Alternate embodiments may use a same value for any of the
increment steps or may use any combination of increment values.
Similarly, the increment values may be adjusted dynamically based
on performance of the system.
FIG. 5 illustrates the power control outer loop, wherein the base
station applies a sawtooth adjustment to the transmission energy
level in response to feedback from the mobile station. The process
is illustrated as a function of time. For a given transmission
between a base station and a mobile station, the base station
adjusts the transmission energy in response to a received EIB (not
shown). Assertion of the EIB corresponds to a frame error
indication, while negation of the EIB corresponds to no frame
error. On assertion of the EIB, the transmission energy is
increased by a predetermined increment value or step size. On
negation of EIB, the transmission energy is decreased by a
predetermined decrement value or step size. At time t.sub.1 a first
frame error is indicated by assertion of the EIB. In response, the
base station increases or boosts the energy level for a next
retransmission. As illustrated, at times t.sub.2, t.sub.3, t.sub.4
the frame errors of the received frames are below the target FER,
and the corresponding EIBs are negated. On occurrence of each EIB
negation, the transmission energy level is decremented by a
predetermined amount. At time t.sub.5 a second frame error is
detected and the corresponding EIB is asserted. In response, the
base station increases the transmission energy. According to one
embodiment, the ratio of the step size is equal to 1/FER. The error
message may be an EIB, or alternately may be a Negative Acknowlege
or NAK signal. Alternate embodiments may implement any signal that
provides information to the base station regarding the transmission
and/or retransmission quality, such as acknowledging the
transmission or acknowledging the transmission was received
incorrectly.
FIG. 6 illustrates the relation between traffic signal strength and
pilot signal strength according to one embodiment. As illustrated,
during a first portion of operation, the ratio of traffic to pilot
is maintained at a first ratio labeled RATIO1. The transmitter may
boost the ratio to RATIO2 in response to inter-frequency hard
handoff or other event. According to one embodiment, a wireless
communication system performs power control of the pilot signal,
such as the RL pilot signal. On occurrence of frame error rates,
the TR/P ratio is adjusted as illustrated in FIG. 6. Once the pilot
signal is adjusted in response to power control, the energies of
transmission and retransmission, respectively, are calculated as
TR/P ratios, while the pilot is maintained at a constant level. As
illustrated in FIG. 6, RATIO1 corresponds to transmission, while
RATIO2 corresponds to retransmission. While the pilot remains at a
constant energy level, the energies of transmission and
retransmission are determined with respect to their relationship
with the pilot energy level. According to one embodiment, power
control is performed on the pilot signal of the RL and the
transmission and retransmission energies are adjusted in response.
The TR/P ratios associated with transmission and retransmission may
be dynamically adjusted with respect to each other in response to
operation of the system. The TR/P ratio(s) are determined to
achieve a target FER.
For implementing outer loop power control, often the target FERs
for transmission and retransmission are determined off-line by
simulation to provide robust, consistent performance over a variety
of operating conditions. In general, the transmission FER, or
FER.sub.1, is not equal to the retransmission FER, FER.sub.2.
FIG. 7 illustrates a method 100 for implementing outer loop power
control at the base station. The process starts at step 102 by
initializing E.sub.1 and E.sub.2 prior to transmission. Default
values for FER.sub.1 and FER.sub.2, as well as E.sub.1 and E.sub.2,
are determined prior to transmission and may be based on
simulations done to optimize performance of the system. A variety
of criteria may be used to determine the default values. In one
embodiment, the default values are used to initiate transmissions,
wherein the values are updated based on feedback from the mobile
station relating to frame errors received. At decision diamond 104,
the base station determines if a frame error message was sent by
the mobile station. If no error message was received, the process
continues to step 108 to decrease the energy level E.sub.1. If an
error message was received at decision diamond 104, the base
station increases the energy level E.sub.1 at step 106. After
E.sub.1 adjustment, processing continues to step 110 to set E.sub.2
equal to E.sub.1 plus a delta value. After a predetermined time
period, processing returns to decision diamond 104 to check for
receipt of an error message. In one embodiment, the frame error
message is an EIB message, wherein the E.sub.1 adjustment is
according to a sawtooth pattern, such as illustrated in FIG. 5. In
this way, the sawtooth adjustment is made to the energy level
E.sub.1 of the first transmission, while the retransmission energy
level E.sub.2 is calculated as a function of E.sub.1. As most
errors occur on the first transmission, the E.sub.1 level is
adjusted first, while a difference is maintained between E.sub.1
and E.sub.2. The difference between E.sub.1 and E.sub.2 may be a
predetermined fixed value, or may be dynamically adjusted as a
function of performance. In one embodiment, E.sub.2 is a function
of E.sub.1, wherein the difference between E.sub.1 and E.sub.2
changes according to the performance of the link.
FIG. 8 illustrates an alternate method 150, wherein both E.sub.1
and E.sub.2 are updated to provide target FER.sub.1 and FER.sub.2,
respectively. The values of E.sub.1 and E.sub.2 are initialized at
step 152. According to one embodiment, the values for FER.sub.1 and
FER.sub.2 are determined off-line by computer simulation using
statistical information regarding operation of the system and the
type of data transmitted. The initialization values of E.sub.1 and
E.sub.2 may also be determined off-line as a function of the
FER.sub.1 and FER.sub.2 values, respectively. At decision diamond
154 the method includes a determination of whether the current
communication is a transmission or a retransmission. On the first
transmission, processing continues to the path of decision diamond
156. If a frame error is detected at decision diamond 156 the
energy setpoint E.sub.1 is increased or incremented at step 158,
else the energy setpoint is decreased or decremented at step 160.
The present embodiment effectively implements a sawtooth adjustment
similar to that illustrated in FIG. 5. The increment and decrement
values may be predetermined fixed value(s) or may be dynamically
adjusted based on operation of the system. In one embodiment the
increment value and the decrement value have equal absolute value.
From steps 158 and 160 the energy setpoint E.sub.1 is updated at
step 162 and after a predetermined time period processing returns
to decision diamond 154 for the next communication. According to
one embodiment, the next communication is the next frame.
Continuing with the method 150 of FIG. 8, for a retransmission,
processing continues from decision diamond 154 to the path of
decision diamond 164. If a frame error is detected at decision
diamond 164, the energy setpoint E.sub.2 is increased or
incremented at step 166, else the energy setpoint is decreased or
decremented at step 168. The present embodiment effectively
implements a separate sawtooth adjustment similar to that
illustrated in FIG. 5 for energy setpoint E.sub.2. The increment
and decrement values may be predetermined fixed value(s) or may be
dynamically adjusted based on operation of the system. In one
embodiment the increment value and the decrement value have equal
absolute value. From steps 166 and 168 the energy setpoint E.sub.2
is updated at step 170 and after a predetermined time period
processing returns to decision diamond 154 for the next
communication.
Note that alternate embodiments may implement multiple
retransmissions, each having an associated FER such as FER.sub.1
and an associated energy setpoint E.sub.1. The values of each
E.sub.1 may be the same as the adjusted value of E.sub.2, or may
each be individually calculated in a processing path similar to
that of decision diamond 164. In one embodiment, the value(s) of
E.sub.1 are calculated as function(s) of E.sub.2, such as to
maintain a predetermined ratio with E.sub.2.
FIG. 9 is a block diagram of an exemplary embodiment of a
transmitting station operative in a wireless communication system.
The information to be transmitted is generated by a data source
302, and is provided to a channel element 304, which partitions the
data, CRC encodes the data, and inserts code tail bits as required
by the system. Channel element 304 then convolutionally encodes the
data, CRC parity bits, and code tail bits, interleaves the encoded
data, scrambles the interleaved data with the user long PN
sequence, and covers the scrambled data with a Walsh sequence. The
channel element 304 then provides the covered data to a gain stage
306, which scales the data in response to a signal from a processor
308, such that the data with required energy E.sub.1 is provided to
a transmitter 310. The transmitter 310 spreads the scaled data with
the short PN.sub.1 and PN.sub.Q sequences. The spread data is then
modulated with the in-phase and quadrature sinusoids, and the
modulated signal is filtered, upconverted, and amplified. The
signal is transmitted on the forward channel if the transmitting
station is a base station, or reverse channel if the transmitting
station is a remote station.
The feedback signal from the receiving station is received by an
antenna 314, and is provided to receiver 316. Receiver 316 filters,
amplifies, downconverts, quadrature demodulates, and quantizes the
signal. The digitized data is provided to demodulator 318, which
despreads the data with the short PN.sub.1 and PN.sub.Q sequences,
and decovers the despread data with the Walsh sequence. The
despread data from different correlators within demodulator 318 are
combined and descrambled with the user long PN sequence. The
descrambled (or demodulated) data is provided to decoder 320 which
performs the inverse of the encoding performed within channel
element 304. The decoded data is provided to data sink 322, and the
processor 308.
Processor 308 is configured to control gain stage 306 so as to
scale the data to be transmitted to a power. Processor 308 is
responsive to information provided by the decoder 320, whether the
transmission was received at the receiving station without error.
Processor 308 further controls the data source 302 together with
the channel element 304, and the gain stage 306 to re-transmit
information frames that had been received in error with the next
available energy.
FIG. 10 is a flowchart showing load estimation in accordance with
one wireless system, such as illustrated in FIG. 9. Flow begins in
block 202 in which the transmitting station evaluates FER as a
function of energy. In one embodiment, the transmitting station
adaptively evaluates feedback information received from the
receiving station. In another embodiment, the transmitting station
evaluates conditions of a transmission channel, e.g., attenuation,
fading, number of multipaths, relative velocity of the RS and the
BS, data rate. The transmitting station then uses a look-up table,
containing simulated FER as a function of energy for all potential
channel conditions, to select the proper relationship for given
conditions.
In block 204, the transmitting station reads the required FER. In
block 206, the transmitting station evaluates the transmission
energy for initial transmission E.sub.1, and potential
retransmissions E.sub.2 , . . . , E.sub.N, in accordance with the
principles outlined above. Thus, the transmitting station can use a
pre-computed solution in the form of a look-up table when
appropriate, or algorithms solving by analytical or numerical
methods.
In block 208, the transmitting station transmits a frame of the
information with transmission energy set to a value of E.sub.1. In
block 210, the transmitting station evaluates whether the
information frame transmitted was received without error. If the
report from the receiving station is positive, the flow restarts in
block 202. If the report from the receiving station is negative,
the transmitting station evaluates in decision diamond 212, whether
there is another transmission energy E.sub.2 , . . . E.sub.N. If
the result of evaluation is positive, the transmitting station
continues in block 214, by re-transmitting information frames that
had been received in error with the next available energy, and the
flow returns to block 210. If the result of evaluation is positive,
the transmitting station reports the failure to a higher level
algorithm in block 216, and the flow continues in block 202.
Note that the methods illustrated in FIGS. 7 and 8 are also
applicable to a system such as illustrated in FIG. 9. Software to
adjust the energy setpoint(s) may be stored in the processor 308 or
may be stored in an alternate memory storage location (not shown).
The adjusted energy setpoint(s) are transmitted to a remote station
via transmitter 310 and antenna 312. The error message(s), such as
an EIB message or a frame error indicator, etc., is received by
receiver 316 via antenna 314.
According to one embodiment, the initializations of the
transmission energy setpoint E.sub.1 and the retransmission energy
setpoint E.sub.2 are performed by processor 308. Similarly,
processor 308 determines if an error message was received from a
remote station, and increases or decreases E.sub.1 in response. The
processor 308 also adjusts E.sub.2 in response to E.sub.1. The
delta value may be determined by processor 308 or may be stored in
a memory storage device (not shown).
According to another embodiment, the processor 308 adjusts the
transmission setpoint E.sub.1 on the first transmission, and
adjusts the retransmission setpoint E.sub.2 on the retransmission.
In this embodiment, the processor 308 determines if the current
communication is a transmission or a retransmission. For a
transmission, if a frame error is received, the processor 308
increases E.sub.1 else the processor 308 decreases E.sub.1. For a
retransmission, if a frame error is received, the processor 308
increases E.sub.2 else the processor 308 decreases E.sub.2. The
increment and decrement values may be predetermined to a fixed
value or may dynamically adjust based on performance of the system
or some other criteria. In this embodiment, the processor 308
adjusts each energy setpoint E.sub.1 and E.sub.2 separately,
wherein the E.sub.2 adjustment is not necessarily a function of the
E.sub.1 adjustment. In one embodiment, the E.sub.1 and E.sub.2
adjustments are done according to a sawtooth adjustment such as
illustrated in FIG. 5.
In one embodiment power control is implemented at the physical
layer. The physical layer implementation provides speed for
retransmission adjustment. As the physical layer implements the
processes instructed by higher layers, it is not easy to keep track
of transmission and/or retransmission quality. In an alternate
embodiment, power control is performed at the RLP layer which is
better adapted for the bookkeeping involved in tracking
transmission and/or retransmission quality. The RLP layer
introduces a delay in the processing and therefore is not able to
adjust the energy setpoints as accurately.
Those of skill in the art would understand that information and
signals may be represented using any of a variety of different
technologies and techniques. For example, data, instructions,
commands, information, signals, bits, symbols, and chips that may
be referenced throughout the above description may be represented
by voltages, currents, electromagnetic waves, magnetic fields or
particles, optical fields or particles, or any combination
thereof.
Those of skill will further appreciate that the various
illustrative logical blocks, modules, circuits, and algorithm steps
described in connection with the embodiments disclosed herein may
be implemented as electronic hardware, computer software, or
combinations of both. To clearly illustrate this interchangeability
of hardware and software, various illustrative components, blocks,
modules, circuits, and steps have been described above generally in
terms of their functionality. Whether such functionality is
implemented as hardware or software depends upon the particular
application and design constraints imposed on the overall system.
Skilled artisans may implement the described functionality in
varying ways for each particular application, but such
implementation decisions should not be interpreted as causing a
departure from the scope of the present invention.
The various illustrative logical blocks, modules, and circuits
described in connection with the embodiments disclosed herein may
be implemented or performed with a general purpose processor; a
Digital Signal Processor, DSP; an Application Specific Integrated
Circuit, ASIC; a Field Programmable Gate Array, FPGA; or other
programmable logic device; discrete gate or transistor logic;
discrete hardware components; or any combination thereof designed
to perform the functions described herein. A general purpose
processor may be a microprocessor; but in the alternative, the
processor may be any conventional processor, controller,
microcontroller, or state machine. A processor may also be
implemented as a combination of computing devices, e.g., a
combination of a DSP and a microprocessor, a plurality of
microprocessors, one or more microprocessors in conjunction with a
DSP core, or any other such configuration.
The steps of a method or algorithm described in connection with the
embodiments disclosed herein may be embodied directly in hardware,
in a software module executed by a processor, or in a combination
of the two. A software module may reside in Random Access Memory,
RAM; flash memory; Read Only Memory, ROM; Electrically Programmable
ROM, EPROM; Electrically Erasable Programmable ROM, EEPROM;
registers; hard disk; a removable disk; a Compact-Disk ROM, CD-ROM;
or any other form of storage medium known in the art. An exemplary
storage medium is coupled to the processor, such that the processor
can read information from, and write information to, the storage
medium. In the alternative, the storage medium may be integral to
the processor. The processor and the storage medium may reside in
an ASIC. The ASIC may reside in a user terminal. In the
alternative, the processor and the storage medium may reside as
discrete components in a user terminal.
The previous description of the disclosed embodiments is provided
to enable any person skilled in the art to make or use the present
invention. Various modifications to these embodiments will be
readily apparent to those skilled in the art, and the generic
principles defined herein may be applied to other embodiments
without departing from the spirit or scope of the invention. Thus,
the present invention is not intended to be limited to the
embodiments shown herein but is to be accorded the widest scope
consistent with the principles and novel features disclosed
herein.
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